The present invention relates to a cathode ray tube having an improved cathode structure, and particularly to a cathode ray tube having an electron gun having a plurality of cathodes supported within respective eyelets of a glass-bonded cathode support assembly which is in turn fixed within and to a cup-shaped first grid electrode, and capable of retaining the insulation strength of the glass-bonded cathode support assembly for a long period of time.
Generally, color cathode ray tubes such as a color picture tube and a color display tube comprise a phosphor screen formed on an inner surface of a faceplate of a panel portion of an evacuated envelope, a shadow mask having a multiplicity of electron beam apertures and spaced from the phosphor screen within the panel portion, an electron gun of the in-line type housed within a neck portion of the evacuated envelope, and a deflection yoke mounted around a funnel portion of the evacuated envelope.
In the operation of the color cathode ray tube, three electron beams emitted from the electron gun are deflected by the deflection yoke and thereafter are projected onto picture elements of the corresponding colors of the phosphor screen through the electron beam apertures of the shadow mask to display a desired color image on the phosphor screen.
FIGS. 6A and 6B are constitutional views showing an example of the constitution of an electron gun used for a conventional color cathode ray tube, FIG. 6A being a side view thereof, and FIG. 6B being a top view thereof.
In FIGS. 6A and 6B, reference numeral 30 designates a cathode; 31 a cup-shaped first grid electrode (G1); 32 a second grid electrode (G2); 33 a third grid electrode (G3); 34 a fourth grid electrode (G4); 35 a fifth electrode (G5); 36 a sixth grid electrode (G6); 37 a shield cup; 38 multiform glass rods; 39 a stem; and 40 an electron gun. In this case, only one cathode 30 is shown, but actually there are three electron guns arranged in a line.
The cathodes 30, the cup-shaped first grid electrode 31, the second grid electrode 32, the third grid electrode 33, the fourth grid electrode 34, the fifth grid electrode 35, the sixth grid electrode 36, and the shield cup 37 are mounted, in order named from the stem 39, in spaced relationship between the pair of multiform glass rods 38 spaced from each other and arranged parallel with each other. This beaded assembly is supported on the stem 39.
The cathodes 30, the cup-shaped first grid electrode 31, the second grid electrode 32, the third grid electrode 33, the fourth grid electrode 34, the fifth grid electrode 35, the sixth grid electrode 36, and the shield cup 37 are supported on and secured to the pair of multiform glass rods 38 through electrode supports. The electron gun 40 is held in place within the neck portion in such a manner that the stem 39 mounting the electron gun 40 thereon is heat-sealed to the open end of the neck portion of the color cathode ray tube, and bulb spacer contacts 200 welded to the shield cup 37 hold the forward end of the electron gun 40 centered in the neck portion.
FIGS. 7A and 7B are a plan view and a sectional view, respectively, showing the constitution of a portion supporting cathodes of an electron gun 40 proposed in Japanese Patent Application Laid-Open No. 56-109429 Publication. Cathodes 30B, 30G and 30R composed of a cathode cap 150 provided with an electron-emissive surface, a cathode sleeve 151, and a skirt portion 152 are secured to an insulating substrate 120b by metal supports 170. The insulating substrate 120b is secured within the cup-shaped first grid portion 31 (FIG. 6A) by a metal member 110.
Normally, when the color cathode ray tube is operated for a long period of time, metal components are evaporated from cathodes 30G, 30B and 30R, and the evaporated metal deposit on the constituent parts arranged in proximity to the cathodes. It has been known that, where the cathodes 30G, 30B and 30R are, for example, oxide cathodes, evaporated metals are magnesium mg from a cap-shaped base metal 150 containing Mg serving as a reducing agent, a chrome (Cr) from a metal sleeve 151. These metals deposit on the surface of the insulating substrate 120b with time of operation and deteriorate the insulation strength.
In FIG. 7B, in order to prevent the insulation strength of the electron gun from deteriorating, the top surface of the insulating substrate 120b is made slightly little higher than the upper open end of the metal member 110, and a step 130 is provided between the top surface of the insulating substrate 120b and the edge of the opening of the metal member.
With the above described constitution, even if the metals evaporated from the three cathodes 30G, 30B and 3OR fall on the top surface of the insulating substrate 120b beyond the metal support 170 and contaminate the top surface of the substrate 120b with time of operation, the vertical part of the step 130 provided between the top surface of the substrate 120b and the upper open end of the metal member 110 is a shadow zone not irradiated by the evaporated metals such that the contamination by metals do not develop in the step 130, and the insulation strength is secured between the metal support 170 and the metal member 110.
Further, in the constitution described in Japanese Patent Application Laid-open No. 56-109429 Publication, there is provided the following arrangement for preventing the deterioration of the electric insulation characteristics caused by the evaporation or sputtering of material constituting the cathode. In FIG. 7A, a line-of-sight passing through the upper end of the metal support 170 from the edge of the cathode cap 150 is designed so as to strike a point A as illustrated in FIG. 7A which is on the edge of the opening of the metal member 110 and is equidistant from the two adjacent cathodes 30B and 30G. The following is a relationship required in this case:
H=(Lxe2x88x92LA)xc3x97HA÷(Lxe2x88x92LB)
where H is a protruding height of the metal support 170 beyond the insulating substrate 120b,
L={(S/2)2+LL2}0.5≈S/20.5,
LL is a distance from the center of the cathodes 30B, 30G and 30R to the edge of the opening of the metal member 110,
LA is a radius of the metal support 170, and
LB is a radius of the cathode cap 150.
However, the invention described in Japanese Patent Application Laid-open NO. 56-109429 Publication does not take into account a prevention of occurrence of a leakage path between the adjacent metal supports 170 on the insulating substrate 120b formed directly by the sputtering or evaporation from the two adjacent cathodes 30B and 30G.
The present invention eliminates those problems as noted above, and an object of the invention is to provide a color cathode ray tube having a constitution for preventing the deterioration of the insulation strength between a plurality of cathodes and a first grid electrode or between a plurality of cathodes.
For achieving the aforesaid object, a color cathode ray tube of the present invention comprises at least an evacuated envelope comprising a panel portion having a phosphor screen formed on an inner surface thereof, a neck portion, and a funnel portion connecting the panel portion and the neck portion, a shadow mask spaced from the phosphor screen and suspended within the panel portion, an electron gun housed in the neck portion comprising at least a plurality of cathodes, a cup-shaped first grid electrode spaced from the plurality of cathodes and a plurality of electrodes spaced between the first grid electrode and the shadow cask for generating and directing a plurality of electron beams toward the phosphor screen, and a deflection yoke mounted in the vicinity of the junction between the neck portion and the funnel portion, the plurality of cathodes being supported within a plurality of eyelets corresponding to the number of the cathodes, respectively, the plurality of eyelets comprising a large-diameter upper portion facing the cup-shaped first grid electrode, a small-diameter lower portion for supporting the plurality of cathodes and a funnel portion for connecting the large-diameter portion and the small-diameter portion, and being disposed within and bonded to a tubular cathode support at the large-diameter upper portion thereof by a bonding glass contained within the tubular cathode support, and the tubular cathode support being disposed within and fixed to the first grid electrode, wherein the bonding glass provides a protrusion beyond an upper open end of the tubular cathode support, the protrusion is provided with a depressed step around a top edge thereof, and (1.4 times a beam spacing S between two electron beams from two adjacent ones of the plurality of cathodes divided by an outside radius of the plurality of eyeletsxe2x88x921.4 mm)xe2x89xa7(a height of the plurality of eyelets protruding above a top surface of the bonding glass)xe2x89xa7(0.98 times the beam spacing S divided by the outside radius of the plurality of eyelets 1.4 mm).
According to the constitution of the present invention, the glass-bonded cathode support assembly is provided with a top surface of the bonding crystallized glass protruding beyond the open end of the dish portion of the tubular cathode support and a step of a narrow width around the circumference of the top surface of the crystallized glass. Therefore, even if, in operation of the color cathode ray tube, metal evaporates from the cathodes and flies toward the top surface of the crystallized glass, the evaporated metal does not deposit on a portion of a step which is shadowed, that is, whose unobstructed view is not possible, when the rim of the upper opening of the cathode support is viewed through the edge of the upper end of the eyelet from the cathode, and the insulation strength between the eyelets and the dish portion of the cathode support is sufficiently maintained so that a dark current does not flow between the eyelets and the cathode support regardless of a potential difference between the eyelets and the cathode support.
Further, according to the constitution of the present invention, the crystallized glass has the top surface protruding beyond the open end of the dish portion, and a depressed step of a narrow width is provided around the circumference of the top surface. Therefore, a portion of the protruding crystallized glass is prevented from moving toward and extending over the rim around the opening of the dish portion during operation of firing the crystallized glass and crack does not occur in the crystallized glass or a portion thereof does not chip off.
Further, according to the present invention, the relationship between the height of the eyelet protruding beyond the top surface of the bonding glass and the outside radius of the eyelet is specified as described above, and even if metals evaporate from the cathodes, a leakage path is prevented from forming between the eyelets arranged close to each other for supporting the cathodes.